Method and apparatus for producing a halftone dot by selectively comparing image signals with highlight and shadow reference values or with halftone dot sub-cell reference values

ABSTRACT

A predetermined number of sections of an original corresponding to a predetermined number of halftone dot sub-cells are scanned to generate an image signal for each section. If the image signal for a given section is greater than a highlight level reference value (Hi), the corresponding halftone dot sub-cell is made one of two colors, black or white. If the image signal for the given section is less than a shadow level reference value (S), the corresponding halftone dot sub-cell is made the other of the two colors, white or black. If the image signal for the given section falls between the highlight and shadow level reference values, it is compared witha value (Y) representing the average density of the predetermined number of sections of the original to determine whether the corresponding halftone dot sub-cell should be made black or white. When a binary density image is to be reproduced, the highlight and shadow level reference values are selected to be so close together that none of the image signals falls between them. In another embodiment which improves the fidelity of the reproduced image, the number of halftone dot sub-cells which are determined to be the one of the two colors, black or white, by comparing the image signals with corresponding halftone dot sub-cell reference values is made to be equal to the number of halftone dot sub-cells which are determined to be the one of the two colors, black or white, by comparing each of the image signals with the average density value (Y). When an original contains picture, text, and line images and completely black and white areas, appropriate values of (Hi) and (S) and appropriate values of reproduction process control signals (a) and (b) are stored in a memory and read out as a scanning head scans the corresponding areas on the original.

FIELD OF THE INVENTION

This invention relates to an apparatus for producing a halftone dot inreproducing images, particularly to such an apparatus in which a signalfor recording a halftone dot is obtained by comparing a voltagecorresponding to the density value of a section of an originalcorresponding to a halftone sub-cell area composing the halftone dotwith the voltage of a reference signal such as a halftone sub-cellreference signal.

BACKGROUND OF THE INVENTION

A mechanical halftone screen comprises a plurality of pinholes (thereare several types of pinholes such as square type or chain type)arranged in matrix. A halftone dot generator of an electronic imagereproducing system has a function of electronically generating halftonedots being equivalent to said mechanical halftone screen.

FIG. 1 shows an electronically-generated halftone dot. Precisely, onehalftone dot corresponding to an area (W×L) is composed of a pluralityof halftone dot elements ω₁ l₁ to ω₂₀ l₂₀ (each halftone dot element iscalled a "halftone sub-cell"). The halftone sub-cell is blackened whenthe density value of the corresponding section of an original is morethan a fixed density value indicated as a number in each of the smallestsquares of FIG. 1. Each of the circumferential halftone sub-cells isgiven a higher threshold density level, while each of the centralhalftone sub-cells is given a lower density threshold level.

By the way, FIG. 1 shows only the threshold levels of density of thehalftone sub-cells situated in one quarter of the halftone dot becausethe density distribution is symmetrical about axes A, B and C in theother quarters.

FIG. 2 shows a graph of the density distribution of the quarter areashown in FIG. 1, in which the longitudinal axis represents the halftonedot percentage (%), the lateral axis represents the number of the column(ω) and the parameter represents the number of the row (l). As isobvious from FIGS. 1 and 2, only the central area of a halftone dot isblackened when the density of corresponding area of the original iscomparatively lower, while a greater part of a halftone dot is blackenedwhen the density of corresponding area of the original is comparativelyhigher. In other words, the density of a portion of an original can beexpressed by the occupation rate of the blackened area to the whole areaof a halftone dot.

Basically, the density value of an area of an image can be expressed bythe corresponding voltage within a certain range (for example, for 0 Vto 6 V). To realize that, an image reproducing system sets up an upperlimit (called a "highlight level" hereinafter) and a lower limit (calleda "shadow level" hereinafter); whereby a halftone sub-cell correspondingto an area of an image of which voltage level is equal to or higher thanthe highlight level is recorded in 0% halftone dot density (hereinafter,this state is identified as one wherein the halftone sub-cell iswhitened), while a halftone sub-cell corresponding to an area of theimage of which the voltage level is equal to or lower than the shadowlevel is recorded in 100% halftone dot density (hereinafter, this stateis referred to as one wherein the halftone sub-cell is blackened).

In this case, when the density value of one division of the original,which is submitted to a color computation process, is used for recordingeach of one-sixteenth areas I, II, III . . . XVI corresponds to 55%halftone dot density, the halftone dot is actually recorded as shown asa hatched area in FIG. 1.

FIGS. 3(b), (c) and (d) show reproduction images of an original shown inFIG. 3(a) recorded according to the density distribution pattern ofFIG. 1. In FIGS. 3(b), (c) and (d), the areas identified by I, II, III .. . XVI of each reproduction image correspond to the areas identified bythe same signs in FIG. 1. These areas are recorded according to thedensity distribution pattern of FIG. 1 by using the density valuesobtained from areas i, ii, iii . . . vii of FIG. 3(a) respectively. Itis now assumed that the original A of FIG. 3(a) is a binary densityimage (consisting of a white portion of 0% halftone dot density and ablack (hatched) portion of 100% halftone dot density), and the averagedensity values of the areas i, ii and iii correspond to halftone dotdensity values of 21%, 55% and 85% respectively. Because the area I iswhitened under a condition in which the density of the correspondingarea of the original is less than 56% halftone dot density, all thehalftone sub-cells thereof are whitened by the signal from the area Ihaving 21% halftone dot density value.

Because the area II includes halftone sub-cells which are to beblackened when the density of the corresponding area of the original hasmore than 20% halftone dot density, about half of the halftone sub-cellsthereof are blackened by the signal of 55% halftone dot density.Similarly, because the area III has the same density distribution asthat of the area II, all the halftone sub-cells are blackened by thesignal of 85% halftone dot density. Consequently, the reproduction imageof FIG. 3(b) is obtained. In this case, the areas I and II take shapesthat are quite different from the corresponding areas i and ii of theoriginal.

Then, assuming that the hatched area of the original A has 50% halftonedot density value, the areas i, ii and iii have halftone dot densityvalues of 10.5%, 27.5% and 42.5% respectively. Consequently, thereproduction image of FIG. 3(c) is obtained, wherein areas I and II aredifferent from the corresponding areas i and ii of the original.

Further assuming that the hatched area of the original A has 80%halftone dot density, and the lower density area has 25% halftone dotdensity value, the areas i, ii and iii have halftone dot density valuesof 36.55%, 55.25% and 71.75% respectively while each of areas v, vi andvii has halftone dot density of 25%. Consequently, the reproductionimage of FIG. 3(d) is obtained, which image is different from theoriginal.

To resolve the above problem, the pickup area (one division) of whichdensity signal is to be submitted to a color computation process at atime should be smaller. However, the reduction of the pickup arearesults in prolongation of the color computation time.

Although the color computation time can be reduced by outputting thedensity signals from a plurality of pickup areas at a time, however, itrequires as many color computation units as that of the pickup areas toembody the above method because each of the computation units is capableof processing the density signal of one pickup area at a time.

Japanese Kokai No. 55-146582 discloses the method as shown in FIG. 14.When an image of a division 103 composed of a plurality of sections (forexample 4×4 sections) arranged in matrix is reproduced, at first severaldot patterns 101 are prepared beforehand as shown in FIG. 14(b). Thearea of each of the dot patterns 101 corresponds to that of one division103 shown in FIG. 14(a), while each of the dot patterns 101 correspondsto a pixel of a certain density range. Then a reproduction image isrecorded as in the following way.

For example, when the density value of a section P₂₂ of the division 103is (5/16)×100%, a dot pattern 5 corresponding to the density isselected. Then the signal of the corresponding square P'₂₂ is output torecord the corresponding halftone sub-cell P"₂₂ of a photosensitivematerial. Meanwhile, when a section P₁₄ (having a density value of(7/16)×100%) is to be recorded, the corresponding halftone sub-cell P"₁₄is recorded by using the dot signal of the corresponding square P'₁₄ ofa dot pattern 7.

Therefore, in this method, a certain number of dot patterns must beprepared for density variation of the divisions of an original. Inaddition, the method has a drawback that even the density of a sectionis high enough, the corresponding halftone sub-cell is not blackenedexcept for a case when the corresponding halftone dot pattern has ablackening dot at the corresponding place.

Japanese Kokai No. 51-114133 discloses a method as shown in FIG. 15. Atfirst, a signal (A) of the density value of each section of an original(shown in FIG. 15(a)) is added to a signal (B) of corresponding halftonesub-cell reference value (shown in FIG. 15(b)) to obtain a signal (C)(shown in FIG. 15(c)). Then by comparing the signal of (C) with a signal(E) of a threshold value (in this case, (E)=19.5) a signal (D) isobtained, expressed by a combination of values "1" (when (C)>(E)) and"0" (when (C)<(E)). This signal is used for controlling a recordingbeam.

The above method aims to prevent a moire pattern from appearing onto areproduction image, therefore, the object of the method is differentfrom that of this invention.

SUMMARY OF THE INVENTION

This invention is proposed to resolve the aforementioned conventionalproblems.

An object of this invention is to increase the resolution of boundaryportions of an image to be reproduced.

In order to attain the above object, this invention provides anapparatus in which an area of an original, that is, a division, is madeto comprise a certain number of sections integrally corresponding to thenumber of the sub-cells composing one halftone dot area, thereby ahalftone dot is recorded by comparing the voltage level of the densitysignal of each of the sections with that of a fixed reference voltage.In the above, the division corresponds to an area of the original ofwhich density signal is processed at a time by a color computationdevice of an image reproducing system.

In the first embodiment of this invention, a voltage corresponding tothe density value of a section is compared with a pre-determined upperlimit voltage, i.e., a highlight level voltage and a lower limitvoltage, i.e., a shadow level voltage of a halftone sub-cell referencesignal. In the second embodiment of this invention, the voltagecorresponding to the density value of a section is compared with thehighlight level voltage, the shadow level voltage and the voltage of thesub-cell reference signal itself.

In both embodiments, for example, when a positive reproduction image isto be recorded by using the density signal of a positive original, thedensity signal of a section which has a corresponding voltage higherthan the highlight level voltage causes the corresponding halftonesub-cell to be whitened (not blackened), while the density signal of asection which has a corresponding voltage lower than the shadow levelvoltage causes the corresponding halftone sub-cell to be blackened.Meanwhile, the density signal of a section which has a correspondingvoltage in between the highlight level voltage and the shadow levelvoltage causes the corresponding halftone sub-cell to be recordedaccording to a conventional halftone dot producing method, that is, amethod in which a halftone dot is produced by comparing the voltagecorresponding to the density signal of a division with the voltage of asub-cell reference signal.

The first embodiment is capable of producing an image of high resolutionwhen the voltage corresponding to the density value of a section ishigher than the highlight level voltage or lower than the shadow levelvoltage. However, the first embodiment is capable of reproducing nohigher quality image than can be reproduced by a conventional method.

In comparison to the above first embodiment, in the second embodiment, avoltage corresponding to the density value of a section between thehighlight level voltage and the shadow level voltage can be comparedwith the voltage of the halftone sub-cell reference signal in order toblacken or whiten the corresponding halftone sub-cell. In thisembodiment, the number of the blackened (whitened) sub-cells is made tobe equal to that obtained by comparing the voltage corresponding to thedensity signal of the same section with the shadow (highlight) levelsignal.

The above and other objects and features of this invention can beappreciated more fully from the following detailed description when readwith reference to the acompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a halftone dot sub-cell density distribution pattern.

FIG. 2 shows a graph of the density distribution shown in FIG. 1.

FIG. 3(a) shows an original. FIGS. 3(b)-(d) show images thereofreproduced by the prior art when the hatched and non-hatched portions ofthe original represent densities corresponding respectively to 100% and0%, 50% and 0%, and 80% and 25% of halftone dot percentage respectively.

FIG. 4 shows an input device of the system of this invention.

FIG. 5 shows a conventional halftone dot generator.

FIG. 6 shows an embodiment of the system of this invention.

FIG. 7 shows the characteristic of the output of the embodiment shown inFIG. 6.

FIGS. 8(a)-(b), 8(c) show images of the original as shown in FIG. 3(a),reproduced by the first embodiment of the present invention andcorresponding respectively to the images reproduced by the prior art asshown in FIGS. 3(b) and 3(c), while FIG. 8(d) shows the image reproducedby the second embodiment of the present invention and corresponding tothe image as shown in FIG. 3(d).

FIG. 9 shows another embodiment of the system of this invention.

FIG. 10 shows a device for counting the number of halftone sub-cells.

FIG. 11 shows a device for correcting the number of blackened halftonesub-cells.

FIG. 12 shows a timing chart of the system shown in FIG. 9.

FIG. 13 shows a flip-flop circuit used in the embodiment shown in FIG.9.

FIGS. 14(a)-14(c) show a conventional method of recording halftonesub-cells.

FIGS. 15(a)-15(d) show a conventional method of recording halftonesub-cells for preventing the phenomnon of moire from taking place onto areproduction image.

PREFERRED EMBODIMENT OF THE INVENTION

FIG. 4 shows an optical system for obtaining a density (image) signal ofeach of the sections of an original. A beam transmitted from orreflected at an original A mounted on an input drum and scanned in main(13) and subscanning (14) directions is refracted by a pickup lens 2 tobe brought to an aperture 3 which adjusts the pickup area. Furthermore,the beam is brought via a lens 4, half mirrors 5 and 6 and B, G and Rcolor filters 7, 8 and 9 to photo-sensors 10, 11 and 12.

The photo-sensors can be a two-dimensional CCD (Charged Coupled Device)photo-sensor or the same wherein the sensory area corresponds to each ofAreas i, ii . . . of the original A as shown in FIG. 3(a) as well as toeach of Areas I, II . . . of the reproduction image B as shown in FIG.3(b). The lenses 2 and 4 are adjusted in order to make the pickup areaand the sampling area of the photo-sensor have a relation of a desiredmagnification ratio M between each other of both the main and thesub-scanning direction factors. This kind of adjustment work can becarried out by varying the sampling area of the photo-sensor.

The elements C₁₁ to C₅₅ of the CCD photo-sensor correspond to thehalftone sub-cells ω₁ l₁ to ω₅ l₅ as well as to sections ω₁ l₁ to ω₅ l₅of an original. Purporting to the photo-sensor, a color CCD photo-sensorcomprising a color filter composed of three elements for componentcolors R, G and B arranged in matrix can be used.

Of course the sampling process can be performed by using amono-dimensional CCD photo-sensor in combination with line memories.Yet, a conventional method for sampling the image signal of each sectionone by one can also be adopted.

FIG. 5 shows an apparatus of this invention for producing a halftone dotby using thus obtained density signal of a section of an original. Thedensity signal of a section is amplified to have a certain amplitude andinput to a halftone dot generator 22 (via an analog latch at need). Thehalftone dot generator 22 generates a halftone dot signal by comparingthe voltage corresponding to the density value of each section to ahalftone sub-cell reference voltage as mentioned afterwards. Meanwhile,the density signal of the section is input to a color computation device21, which carries out a color computation process on the density signalobtained by averaging the sum of the density values from the sensorelements of the photo-sensor 10 (11, 12) of which sensory areacorresponds to a plurality of sections (in this case, 25 sections (=onedivision)), and outputs a resultant of color separation signals Y(Yellow), M (Magenta), C (Cyan) and K (Black) to said halftone dotgenerator 22.

The halftone dot signal obtained in the halftone dot generator 22 isonce stored into a memory 23 and then input to, for example, a laserbeam producer 24, which exposes a photosensitive material 26 beingscanned in main (27) and subscanning (28) directions according to thehalftone dot signal by means of a laser beam via a laser beam lens 25.

The memory 23 is ordinarily used in any scanners for the purpose ofcontinuously varying the magnification ratio between the input side andthe output side, however, no detailed explanation is prepared for thedevice here because it is not the main factor of this invention.

FIG. 6 shows an embodiment of the halftone dot generator 22, wherein acircuit for one of the separation colors of Y, M, C and K represents theother three (not indicated). The following explanation is based on thecircuit for reproducing Y color separation image. Since the colorcomponent signal B corresponds to the color separation image Y, thesignal from each of the elements C₁₁ to C₅₅ of the photo-sensor 10 for Bcolor component is input directly to the halftone dot generator 22 asthe yellow density signal of the corresponding section. The signal isamplified and controlled of its input timing by means of a latch atneed.

The density signals from the sensor elements C₁₁ to C₁₅ are input to aselector switch 36₋₁. Likewise, the density signals from the sensorelements C₂₁ to C₂₅, C₃₁ to C₃₅, C₄₁ to C₄₅ and C₅₁ to C₅₅ are input toselector switches 36₋₂, 36₋₃, 36₋₄ and 36₋₅ respectively. The selectorswitch 36₋₁ selects the signals in order of the signals from C₁₁, C₁₂,C₁₃, C₁₄ to C₁₅. Likewise, the selector switches 36₋₂, 36₋₃, 36₋₄ and36₋₅ selects the signals in the same manner.

The output signals of the selector switches 36₋₁ to 36₋₅ are input tothe positive terminals of comparators 32₋₁ to 32₋₅ as well as to thepositive terminals of the comparators 33₋₁ to 33₋₅ respectively. On theother hand, a highlight level voltage signal (Hi) (+5 V corresponding to0% halftone dot density value) is input to the negative terminals of thecomparators 33₋₁ to 33₋₅, while a shadow level voltage signal (S) (+2 Vcorresponding to 100% halftone dot density value) is input to thenegative terminals of the comparators 32₋₁ to 32₋₅.

As is obvious from FIG. 6, each of the comparators 31, 32 and 33, anAND-gate 34 and a NOR-gate 35 comprises individually operating fiveunits, however, the following only describes the first units of eachcircuit in order to give a simple explanation of their functions. When avoltage corresponding to the density value from the sensor element C₁₁(C₁₂, C₁₃, C₁₄ or C₁₅) is higher than that of the highlight levelvoltage (for example, when the former is the voltage of +6 V), thecomparator 33₋₁ outputs "H" (High level) signal to a NOR-gate 35₋₁.Therefore the NOR-gate 35₋₁ outputs "L" (Low level) signal regardless ofthe output of an AND-gate 34₋₁. This condition is called " ○Bcondition", which means that a signal for whitening the correspondinghalftone sub-cell (in other words, the corresponding halftone sub-cellis not blackened by this signal) is output.

When a voltage corresponding to the density value from the sensorelement C₁₁ (C₁₂, C₁₃, C₁₄ or C₁₅) is lower than that of the shadowlevel voltage (for example, when the former is the voltage of +1 V), thecomparator 32₋₁ outputs "L" signal to the AND-gage 34₋₁. Therefore theAND-gate 34₋₁ outputs "L" signal regardless of the output of acomparator 31₋₁ (mentioned afterwards) to one terminal of the NOR-gate35₋₁. While "L" signal is input from the comparator 33₋₁ to the otherterminal of the NOR-gate 35₋₁ because the input voltage is lower thanthe shadow level voltage, so the NOR-gate 35₋₁ outputs "H" signal. Thiscondition is called " ○C condition", which means that a signal forblackening the corresponding halftone sub-cell is output.

When a voltage corresponding to the density value from the sensorelement C₁₁ (C₁₂, C₁₃, C₁₄ or C₁₅) is in between the highlight and theshadow level voltages (for example a signal of 50% density valuerepresented by the voltage of 3.5 V), the halftone dot generator 22operates as follows. A density signal Y obtained by averaging thedensity values from the sensor elements C₁₁ to C₅₅ via the colorcomputation device 21 is input to the positive terminal of thecomparator 31₋₁, while halftone dot sub-cell reference signals for thehalftone dot sub-cells ω₁ l₁ -ω₁ l₅, ω₂ l₁ -ω₂ l₅, ω₃ l₁ -ω₃ l₅, ω₄ l₁-ω₄ l₅ and ω₅ l₁ -ω₅ l₅ as shown in FIG. 1 are successively input to thenegative terminals D₁ -D₅ of comparators 31₋₁ -31₋₅ respectively.Therefore, when the voltage of the density signal Y is higher than thatof the corresponding halftone sub-cell reference signal D1, thecomparator 31₋₁ outputs "H" signal to the AND-gate 34₋₁. When the formeris lower than the latter, the comparator 31₋₁ outputs "L" signal to theAND-gate 34₋₁. On the other hand, the comparator 32₋₁ outputs "H" signalto the AND-gate 34₋₁, while the comparator 33₋₁ outputs "L" signal tothe NOR-gate 35-1. Therefore, when the comparator 31₋₁ outputs "H"signal, the NOR-gate outputs "L" signal. When the comparator 31₋₁outputs "L" signal, the NOR-gate outputs "H" signal. This condition iscalled " ○A condition", which means that "H" signal blackens thecorresponding halftone sub-cell while "L" signal whiten thecorresponding halftone sub-cell (in other words, the corresponding areaof a photosensitive material is blackened or whitened).

FIG. 7 shows a relation between the density value (voltage level) of theoriginal and the halftone dot percentage of a reproduction image when apositive reproduction image is obtained from a positive original.

The embodiment of FIG. 6 is a circuit for obtaining a positive original,therefore, in order to produce a negative reproduction image from apositive original, OR-gate units must be substituted for the NOR-gateunits 35₋₁, 35₋₂, 35₋₃, 35₋₄ and 35₋₅.

The image of the original A shown in FIG. 3(a) is reproduced by means ofthe halftone dot generator shown in FIG. 6 under the condition indicatedin FIG. 7 as in the following way.

FIG. 8(a) is a reproduction image of the original A shown in FIG. 3(a)obtained by using the circuit of FIG. 6 under a condition in which theshadow level voltage is +2 V while the highlight level voltage is +5 V.Assuming that the voltages corresponding to the density values of thesections ω₁ l₁ and ω₁ l₂ of original A as shown in FIG. 3(a) are inbetween the highlight and the shadow level voltages (for example, whenthey are +2.2 V and +2.1 V respectively), the halftone dot generator 22operates in ○A condition. Meanwhile, to the negative terminal of thecomparator 31₋₁, the halftone dot sub-cell reference signals havingvoltage +2 V and +2.06 V corresponding to the densities 100% and 98% ofthe halftone dot sub-cells ω₁ l₁ and ω₁ l₂ are input successively. Onthe other hand, to the positive terminal of the comparator 31₋₁, thedensity signal Y of voltage +4.37 V (21% halftone dot density) of thecorresponding division i of the original A is input. Therefore, thecomparator 31₋₁ outputs an "H" signal in response to each of saidhalftone sub-cell reference signals. Consequently, the NOR-gate 35-1outputs "L" signals successively to whiten the halftone sub-cells ω₁ l₁and ω₁ l₂, as shown in FIG. 8(a).

The halftone sub-cells ω₁ l₃ to ω₁ l₅, as shown in FIG. 8(a), areblackened because the voltage of each of the corresponding sections ω₁l₃ to ω₁ l₅ of the original A as shown in FIG. 3(a) is less than 2 V(that is, the halftone generator 22 operates in ○C condition).

Inasmuch as the voltage corresponding to the density value of each ofthe sections ω₁ l₈ and ω₂ l₈ as shown in FIG. 3(a) is less than theshadow level voltage +2 V (the halftone dot generator operates in ○Ccondition), the corresponding halftone sub-cells ω₁ l₈ and ω₂ l₈ areblackened, as shown in FIG. 8(a). Voltage corresponding to the densityvalue of a section ω₃ l₈ shown in FIG. 3(a) is now in between thehighlight level voltage and the shadow level voltage (the halftone dotgenerator 22 operates in ○A condition). In this, a signal of voltage+3.35 V corresponding to 55% halftone dot density of the section ω₃ l₈shown in FIG. 3(a) is input to the positive terminal of the comparator31₋₃, and, as shown in FIG. 1, is compared with the voltage +3.5 Vcorresponding to 50% halftone dot density being input to the negativeterminal of the comparator 31₋₃. Thus, the comparator 31-3 outputs "L"signal to blacken the halftone sub-cell ω₃ l₈ as shown in FIG. 8(a).Inasmuch as the voltage corresponding to each of the sections ω₄ l₈ andω₅ l₈ is more than the highlight level voltage as shown in FIG. 3(a),the corresponding halftone sub-cells ω₄ l₈ and ω₅ l₈ are not blackened,as shown in FIG. 8(a).

Thus, the reproduced image as shown in FIG. 8(a) by means of thisinvention achieves, as shown in FIG. 3(a) more fidelity to the originalthan the reproduction image shown in FIG. 3(b) recorded by means of aconventional method. The proportion of the black area to the white areadepends on the range of ○A condition in this case. When a binary densityimage must be reproduced faithfully, the shadow and the highlight levelvoltages had better, for example, be +3.47 V and +3.5 V which correspondto 51% and 50% halftone dot density values respectively. In this case,the output signals of the comparators 31₋₁ to 31₋₅ are hardly affectedby the halftone sub-cell reference signals, consequently a reproductionimage faithful to the original A as shown in FIG. 8(b) can be obtained.

In comparison to the above-mentioned reproduction images of FIGS. 8(a)and (b) composed of solid and white areas, FIG. 8(c) shows areproduction image of the original A composed of 50% halftone dotdensity areas and white portions. In FIG. 3(a), the hatched areacorresponding to voltage +3.5 V is recorded in ○A condition. Because theareas i, ii and iii have halftone dot density values of 10.5%, 27.5% and42.5% respectively, the reproduction image of FIG. 8(c) is obtained as aresult. The reproduction image of FIG. 8(c) is slightly superior to thatof FIG. 3(c) in terms of expressing the border line of the vacant areaand the 50% halftone dot percent area. By the way, a color image isexpressed by using four separation inks Y, M, C and K, therefore, theyalways compensate for each other in forming the border line of theimage.

When the hatched area of the original A has 80% halftone dot densityvalue (+2.75 V) and the low-density area thereof has 25% halftone dotdensity value (+4.25 V), the original A is reproduced in ○A condition aswell as in a conventional way to be a reproduction image of FIG. 3(d)(not shown in FIG. 8).

By comparing the reproduction images of FIGS. 3, (b) and (c) with thereproduction images of FIGS. 8(a), (b) and (c), the followingobservation can be obtained.

That is, when the original is composed of areas of 0% density and areasof 100% density (binary density image), the method of this invention iscapable of producing high-fidelity reproduction images as shown in FIGS.8(a) and (b).

However, the number of the blackened sub-cells of the areas II or III ofthe image of FIG. 8(c) is different from that of the image of FIG. 3(c).Precisely, the area II of FIG. 8(c) comprises one blackened sub-cell asagainst three of FIG. 3(c), while the area III of FIG. 8(c) compriseseight blackened sub-cells as against nine of FIG. 3(c). Generally, thereproduction images of FIG. 3 are superior to that of FIG. 8 in terms oftheir fidelity to the originals. That is because the reproduction imageof FIG. 8 are recorded without using the color computation device.

FIG. 9 shows another embodiment of this invention, which carries out anobject of recording a reproduction image of higher fidelity to itsoriginal when the voltage of a signal from each of the sensor elementsC₁₁ to C₅₅ is in between the highlight level voltage (Hi) and the shadowlevel voltage (S) (i.e. ○A condition).

FIG. 12 shows a timing chart of the circuit of FIG. 9, in which timingpulses T_(P1) to T_(P5) are generated by a timing pulse generator 61using clock pulses obtained from rotary encoders connected to input andoutput drum motors of an image reproducing system.

A halftone sub-cell reference signal generator 67 outputs a halftonesub-cell reference signal D to a comparator 51. The comparator 51comprises twenty-five comparator units 31_(A-11) -31_(A-55). Each unitof the comparator compares the voltage of the corresponding referencesignal with that of a signal output from an image processor (in thiscase, Y color signal of a division). Each of the comparator unitsoutputs to a counter 53 a state signal d₂ which becomes "H" when thevoltage of the signal Y is equal to or higher than that of thecorresponding reference signal, or becomes "L" when the former is lowerthan the latter. The counter 53 counts the number of "L" signalcontained in the state signal d₂ and outputs the number as a signal d₄to a comparator 55 (mentioned afterwards).

In the meantime, the density signals S from the sensor elements C₁₁ toC₅₅ of the CCD photo-sensor 10 are latched in an analog latch 56 insynchronism with the timing pulse T_(P5), and input to a comparator 52.The comparator 52 comprises twenty-five comparator units, each of whichcompares the voltage of one corresponding signal out of the signals fromthe sensor elements C₁₁ to C₅₅ with that of the corresponding referencesignal D. Each of the comparator units outputs a state signal d₁, whichbecomes "H" when the voltage of the density signal is higher than thatof the corresponding reference signal D, or becomes "L" when the formeris lower than the latter.

Incidentally, in the embodiment of FIG. 9, a positive reproduction imageof a positive original is obtained, that is, each halftone sub-cell isblackened by "H" signal output from a gate circuit 64 (mentioned later)when the output of the comparator 52 is "L".

The state signal d₁ is input via a bistable latch 65 being controlled bya control signal i₂ (mentioned afterwards) to a counter 54. The counter54 counts the number of "L" signal component and output the resultantvalue d₃ to the comparator 55, which compares the count value d₃ withthe count value d₄. When d₄ >d₃, the comparator 55 outputs a selectionsignal SE₁ of "H" from its > terminal to a correction circuit 57. Whend₄ <d₃, the comparator 55 outputs a selection signal SE₂ of "H" from its< terminal to the correction circuit 57. The correction circuit 57carries out a correction process on the density signals from the sensorelements C₁₁ to C₅₅ in order to make the value d₄ agree with the valued₃. When d₄ =d₃, the comparator 55 outputs a coincidence signal i₁ of"H" to a flip-flop circuit 66, which outputs the control signal i₂ of"L" to the bistable latch 65. The bistable latch 65 holds the statesignal d₁ under the condition when d₃ =d₄ until the timing pulse T_(P1)is input to the flip-flop circuit 66.

Thus corrected state signal d₁ is input to a P/S converter 60 via gatecircuits 59 and 64 (mentioned afterwards) in synchronism with the timingpulse T_(P2). The P/S converter 60 outputs density signals from onecolumn of the sensor elements (For example, C₁₁ to C₁₅) one by one indue order to a memory 23 in synchronism with the timing pulse T_(P3).

FIG. 10 shows an embodiment of the counter 53. The state signal d₂ islatched in a latch 71 synchronizing with the timing pulse T_(P5) andthen input to an LED matrix 72 which has as many LED units as the sensorelements. A certain number of the LED units to which "L" signals areinput corresponding to the value of the state signal d₂ (the number ofthe halftone sub-cells to be blackened) emit light. The quantity of thelights are detected and converted into said value signal d₄ by aphoto-sensor 73 to be output to the comparator 55.

The counter 54 has the same structure as that of the counter 53,however, a latch corresponding to the latch 71 is not provided becausethe density signals from the sensor elements are input to the latch 56in synchronism with the timing pulse T_(P5).

FIG. 11 shows the detail of said correction circuit 57. The negativeinput terminal and the output terminal of an amplifier areshort-circuited while the timing pulse T_(P4) is "H". When the timingpulse T_(P4) becomes "L", a capacitor 81_(c) begins to be charged,consequently the amplifier 81_(a) outputs a correction signal R₂ havinga forward-declining saw-tooth waveform as shown in FIG. 12.

Meanwhile, an amplifier 81_(b) outputs a correction signal R₁ having aforward-uprising saw-tooth waveform as shown in FIG. 12. Then thevoltage of either of the correction signals R₁ or R₂ is added to each ofthe density signals from the sensor elements C₁₁ to C₅₅ to be input tothe positive terminals of the corresponding comparator units 31_(A-mn)respectively.

Precisely, when d₄ >d₃, the selection signal SE₁ of "H" is output from >terminal of the comparator 55, which signal closes a switch 86_(mn) toreduce the voltage applied to the positive terminal of each of thecomparator units until the value d₃ (the number of the comparator unitsof which outputs are "L") comes up to the value d₄.

When d₄ <d₃, the selection signal SE₂ of "H" is output from < terminalof the comparator 55, which signal closes a switch 85_(mn) to increasethe voltage applied to the positive terminal of each of the comparatorunits until the value d₃ (the number of the comparator units of whichoutputs are "L") comes down to the value d₄.

In this, the number of the switches 85_(mn) (86_(mn)) is the same as thecomparator units.

FIG. 13 shows the detail of the flip-flop circuit 66. The flip-flopcircuit 66, composed of two NOR-gates 91 and 92 and a NOT-gate 93 behindthe NOR-gate 91, outputs the control signal i₂ of "H" when the timingpulse T_(P1) is input thereto or outputs the control signal i₂ of "L"when the coincidence signal i₁ is input thereto. Therefore, the densitysignals from the sensor elements C₁₁ to C₅₅ held in the latch 56according to the timing pulse T_(P5) are latched in the latch 65 as acorrected state signal d₁ in conformity with the control signal i₂ whichchanges from "H" to "L" the moment a correction process is completed.Then the control signal i₂ becomes "H" again synchronizing with thetiming pulse T_(P1) after when the state signal d₁ is input to a P/Sconverter in accordance with the timing pulse T_(P2).

In FIG. 3(a), when the higher density area has 80% halftone dot densityand the lower density area has 25% halftone dot density, in other words,when the density signals from the sensor elements C₁₁ to C₅₅ are inbetween the highlight level and the shadow level, the corrected statesignal d₁ is obtained as in the following way.

When the original of FIG. 3(a) is under the above-mentioned condition,the output signal (the condition signal d₂) of the comparator 51corresponds to the density distribution shown in FIG. 3(d). Meanwhile,the output signal (the uncorrected condition signal d₁) of thecomparator 52 corresponds to the density distribution shown in FIG.8(d).

Precisely, the sections of higher density area are blackened when thehalftone sub-cell reference signals correspond to more than 80% halftonedot density, while the sections of lower density area are blackened whenthe halftone sub-cell reference signals correspond to less than 25%halftone dot density. Meanwhile, the sections which lie on the boundarybetween both the areas are blackened when the voltage corresponding tothe average density of each of the sections are more than that of thecorresponding halftone sub-cell reference signal.

In the area II of FIG. 3(d), fifteen sections are blackened, while inthe area II of FIG. 8(d), fourteen sections are blackened by comparison.Accordingly, the comparator 55 outputs the selection signal SE₁ of "H"from its > terminal, which signal closes the switch 86_(mn) to reducethe voltage levels corresponding to the density signals from the sensorelements C₁₁ to C₅₅. So the comparator unit having an output voltagewhich is closest to the input level thereof (in this case, the unit31_(A-12)) out of all the comparator units 31_(A-mn) becomes "L" toblacken the corresponding halftone sub-cell (in this case, the sub-cellω₁ l₇). Synchronizing with the blackening of the sub-cell ω₁ l₇, thecomparator 55 outputs the coincidence signal i₁ from its = terminal.Then the flip-flop circuit outputs the control signal i₂ of "L" to thebistable latch 65 to hold the corrected condition signal d₁.

Since the correction signal R₂ is a gradually increasing voltage signal,the halftone sub-cells are blackened in order of ω₁ l₇ →ω₁ l₆ →ω₂ l₆ →ω₂l₈. Similarly, since the correction signal R₁ is a gradually decreasingvoltage signal, the halftone sub-cells are whitened in order of ω₅ l₉→ω₄ l₉ →ω₃ l₈ →ω₅ l₁₀.

In the embodiment of FIG. 9, a comparator 58 comprises twenty-fivecomparator units 32_(B-11) to 32_(B-55), each of which receives thevoltage signal from the corresponding sensor element at its positiveterminal as well as the shadow level signal (S) at its negativeterminal. Each of the comparator units 32_(B-11) to 32_(B-55) has thesame function as each unit of the comparator 32 of FIG. 6. Thecomparator 58 comprises another twenty-five comparator units 33_(B-11)to 33_(B-55), each of which receives the voltage signal from thecorresponding sensor element at its positive terminal as well as thehighlight signal (Hi) at its negative terminal. Each of the comparatorunits 33_(B-11) to 33_(B-55) has the same function as that of each unitof the comparator 33 of FIG. 6.

A gate-circuit 59 has twenty-five AND-gate units 34_(B-11) to 34_(B-55)corresponding to each unit of the AND-gate 34 of FIG. 6, and twenty fiveOR-gate units 35_(B-11) to 35_(B-55) corresponding to each unit of theNOR-gate 35. The comparator 58 and the gate circuit 59 determine therange of ○A , ○B and ○C operation conditions shown in FIG. 7 accordingto the highlight level (Hi) and the shadow level (S) being previouslyset up to a RAM 63 mentioned afterwards. Within ○A operation condition,the bistable latch 65 outputs the image signal.

A gate-circuit 64 comprises twenty-five AND-gate units 34_(C-11) to34_(C-55) corresponding to each unit of the AND-gate 34 of FIG. 6, andtwenty-five NOR-gate units 35_(C-11) to 35_(C-55) corresponding to eachunit of the NOR-gate 35 of FIG. 6.

By the way, recording conditions for each of the color separation imagescan be controlled by inputting output condition selection signals ○a and○b to the AND-gate units 35_(C-11) to 35_(C-55) respectively. When theselection signal ○a is "H" and the selection signal ○b is "L", thepresent color separation film is recorded by the output signal of thegate circuit 59. When the selection signal ○b is "H", the present colorseparation film is recorded with white halftone sub-cells. When theselection signal ○a is "L" and the selection signal ○b is "L", thepresent color separation film is recorded with black halftone sub-cells.

The above explanation is based on a condition in which a positivereproduction image is recorded by using the image signal of a positiveoriginal. So when a negative reproduction image must be recorded byusing the image signal of a positive original, said NOR-gate units35_(C-11) to 35_(C-55) must be replaced by OR-gate units.

A counter 62 produces a signal for designating the present scanningposition by using output pulses from rotary encoders (not shown), andthen outputs the signal to a RAM 63. The RAM 63 is previously providedwith data of the highlight level voltage (Hi), shadow level voltage (S)and the selection signals ○a and ○b corresponding to the portions of anoriginal. The signals of (Hi), (S), ○a and ○b can take appropriatevalues for controlling the reproduction state of each color separationimage and each area thereof. Practically, the signals ○a and ○b can be"H" or "L", while the signal (Hi) or (S) is able to take an arbitraryvoltage within a certain range (for example, 2 V to 5 V).

When a drawing image such as a literal image or a linear design imagemust be reproduced, the signals (Hi) and (S) must take closest voltages,for example +3.5 V and +3.49 V respectively. In this, color separationfilms to be recorded can be designated by the signals ○a and ○b .

Besides, the system of this invention is capable of reproducing anoriginal recorded by means of halftone dots as a binary density image,and is further capable of reproducing an original of a hairline designwith producing no moire pattern.

By setting up appropriate values for the voltage signals (Hi) and (S) tothe RAM 63, an original of a mixture of drawing images and halftone dotimages can be reproduced.

By the way, density signal of each of divisions to be input to the colorcomputation device can be detected by a photo-sensor composed of oneelement.

Although in the aforesaid embodiments, one section corresponds to onehalftone sub-cell, any number of sections of the original can correspondto any number of sub-cells on the reproduced image, having ratios whichmay be, for example, 1:2, 2:1 or 2:3.

As mentioned above, the apparatus of this invention is capable ofrecording a reproduction image of higher resolution, in addition, it iscapable of reproducing a drawing image and a multi-color imageindependently or together.

I claim:
 1. A method of producing a halftone dot comprising apredetermined number of halftone dot sub-cells by making each sub-cell(ω_(m) l_(n)) one of two colors, black or white, comprising the stepsof:(a) obtaining a value (C_(mn)) representative of the density of eachof a predetermined number of sections of an original corresponding tothe predetermined number of halftone dot sub-cells by scanning theoriginal; (b-1) making each halftone dot sub-cell (ω_(m) l_(n)) one ofsaid two colors, black or white, when the corresponding value (C_(mn))is larger than a first reference value (Hi); (b-2) making each halftonedot sub-cell (ω_(m) l_(n)) the other of said two colors, white or black,when the corresponding value (C_(mn)) is smaller than a second referencevalue (S); (b-3) if a value (C_(mn)) is between the first referencevalue (Hi) and the second reference value (S), determining whether thecorresponding halftone dot sub-cell (ω_(m) l_(n)) is to be black orwhite by comparing a value (Y) representative of the average density ofsaid predetermined number of sections of said original with apredetermined reference value for the corresponding sub-cell (ω_(m)l_(n)).
 2. A method as recited in claim 1 in which the first referencevalue (Hi) and the second reference value (S) are selected to be soclose to each other that none of the values (C_(mn)) falls between themso that a binary density image may be faithfully reproduced.
 3. A methodof producing a halftone dot comprising a predetermined number ofhalftone dot sub-cells by making each sub-cell (ω_(m) l_(n)) one of twocolors, black or white, comprising the steps of:(a) obtaining a value(C_(mn)) representative of the density of each of a predetermined numberof sections of an original corresponding to the predetermined number ofhalftone dot sub-cells by scanning the original; (b-1) determining whichof the halftone dot sub-cells should be one of said two colors, black orwhite, by comparing the values (C_(mn)) with corresponding predeterminedhalftone dot sub-cell reference values; (b-2) determining which of thehalftone dot sub-cells should be said one of said two colors, black orwhite, by comparing a value (Y) representative of the average density ofsaid predetermined number of sections of said original with each of saidhalftone dot sub-cell reference values; (b-3) comparing numbers (d3) and(d4) of said halftone dot sub-cells which are determined to be said oneof said two colors, black or white, in steps (b-1) and (b-2),respectively; (b-4) if (d3) is not equal to (d4), determining to be saidone of said two colors, black or white, a halftone dot sub-celldetermined in step (b-1) to be the other of said two colors, white orblack, corresponding to one of said predetermined number of sections ofsaid original having a value (C_(mn)) which is closest to itscorresponding predetermined halftone dot sub-cell reference value ordetermining to be the other of said two colors, white or black, ahaltone dot sub-cell determined in step (b-1) to be said one of said twocolors, black or white, corresponding to one of said predeterminednumber of sections of said original having a value (C_(mn)) which isclosest to its corresponding predetermined halftone dot sub-cellreference value until (d3) is equal to (d4); (b-5) latching a signalrepresenting the color of each of said halftone dot sub-cells asdetermined in steps (b-1) to (b-4); (c-1) making each halftone dotsub-cell said one of said two colors, black or white, when thecorresponding value (C_(mn)) is larger than a first reference value(Hi); (c-2) making each halftone dot sub-cell the other of said twocolors, white or black, when the corresponding value (C_(mn)) is smallerthan a second reference value (S); (c-3) if a value (C_(mn)) is betweenthe first reference value (Hi) and the second reference value (S),making the corresponding halftone dot sub-cell one of said two colors,black or white, in accordance with the signal latched in step (b-5). 4.A method as recited in claim 3 in which the first reference value (Hi)and the second reference value (S) are selected to be so close to eachother that none of the values (C_(mn)) falls between them so that abinary density image may be faithfully reproduced.
 5. A method asrecited in claim 3, wherein said original contains picture images to bereproduced with halftone dots and binary density images such as text andline images and completely black and white areas, said method furthercomprising the steps of:storing in a random-access memory (RAM) thelocations on said original of said picture, text, and line imagestogether with appropriate values of said first and second referencevalues (Hi) and (S) and appropriate values of first and second signals(a) and (b) for controlling recording of said picture, text, and lineimages, and storing in said random-access memory (RAM) the locations onsaid original of said completely black and white areas with appropriatevalues of said first and second signals (a) and (b) for controllingrecording of said completely black and white areas; and reading out saidappropriate values of said first and second reference values (Hi) and(S) and said appropriate values of said first and second signals (a) and(b) from said random-access memory (RAM) when a scanning head scanningsaid original reaches the corresponding locations on said original. 6.An apparatus for producing a halftone dot comprising a predeterminednumber of halftone dot sub-cells by making each sub-cell (ω_(m) l_(n))one of two colors, black or white, said apparatus comprising:(a) densityobtaining means for obtaining a value (C_(mn)) representative of thedensity of each of a predetermined number of sections of an originalcorresponding to the predetermined number of halftone dot sub-cells byscanning the original; (b-1) comparing means being operable for makingeach halftone dot sub-cell (ω_(m) l_(n)) one of said two colors, blackor white, when the corresponding value (C_(mn)) is larger than a firstreference value (Hi); (b-2) said comparing means further being operablefor making each halftone dot sub-cell (ω_(m) l_(n)) the other of saidtwo colors, white or black, when the corresponding value (C_(mn)) issmaller than a second reference value (S); (b-3) if a value (C_(mn)) isbetween the first reference value (Hi) and the second reference value(S), said comparing means further being operable for determining whetherthe corresponding halftone dot sub-cell (ω_(m) l_(n)) is to be black orwhite by comparing a value (Y) representative of the average density ofsaid predetermined number of sections of said original with apredetermined reference value for the corresponding sub-cell (ω_(m)l_(n)).
 7. An apparatus for producing a halftone dot comprising apredetermined number of halftone dot sub-cells by making each sub-cell(ω_(m) l_(n)) one of two colors, black or white, said apparatuscomprising:(a) density obtaining means for obtaining a value (C_(mn))representative of the density of each of a predetermined number ofsections of an original corresponding to the predetermined number ofhalftone dot sub-cells by scanning the original; (b-1) determining meansbeing operable for determining which of the halftone dot sub-cellsshould be one of said two colors, black or white, by comparing thevalues (C_(mn)) with corresponding predetermined halftone dot sub-cellreference values; (b-2) said determining means further being operablefor determining which of the halftone dot sub-cells should be said oneof said two colors, black or white, by comparing a value (Y)representative of the average density of said predetermined number ofsections of said original with each of said halftone dot sub-cellreference values; (b-3) said determining means further being operablefor comparing nunbers (d3) and (d4) of said halftone dot sub-cells whichare determined to be said one of said two colors, black or white, asrecited in (b-1) and (b-2), respectively; (b-4) if (d3) is not equal to(d4), said determining means further being operable for determining tobe said one of said two colors, black or white, a half-tone dot sub-celldetermined as recited in (b-1) to be the other of said two colors, whiteor black, corresponding to one of said predetermined number of sectionsof said original having a value (C_(mn)) which is closest to itscorresponding predetermined halftone dot sub-cell reference value or fordetermining to be the other of said two colors, white or black, ahalftone dot sub-cell determined as recited in (b-1) to be said one ofsaid two colors, black or white, corresponding to one of saidpredetermined number of sections of said original having a value(C_(mn)) which is closest to its corresponding predetermined halftonedot sub-cell reference value until (d3) is equal to (d4); (b-5) saiddetermining means further being operable for latching a signalrepresenting the color of each of said halftone dot sub-cells asdetermined in (b-1) to (b-4); (c-1) comparing means being operable formaking each halftone dot sub-cell said one of said two colors, black orwhite, when the corresponding value (C_(mn)) is larger than a firstreference value (Hi); (c-2) said comparing means further being operablefor making each halftone dot sub-cell the other of said two colors,white or black, when the corresponding value (C_(mn)) is smaller than asecond reference value (S); (c-3) if a value (C_(mn)) is between thefirst reference value (Hi) and the second reference value (S), saidcomparing means further being operable for making the correspondinghalftone dot sub-cell one of said two colors, black or white, inaccordance with the signal latched as recited in (b-5).
 8. An apparatusas recited in claim 7, wherein said original contains picture images tobe reproduced with halftone dots and binary density images such as textand line images and completely black and white areas, said apparatusfurther comprising:random-access memory (RAM) storage means for storingthe locations on said original of said picture, text, and line imagestogether with appropriate values of said first and second referencevalues (Hi) and (S) and appropriate values of first and second signals(a) and (b) for controlling recording of said picture, text, and lineimages, and for storing the locations on said original of saidcompletely black and white areas with appropriate values of said firstand second signals (a) and (b) for controlling recording of saidcompletely black and white areas; and reading means for reading out saidappropriate values of said first and second reference values (Hi) and(S) and said appropriate values of said first and second signals (a) and(b) from said random-access memory (RAM) storage means when a scanninghead scanning said original reaches the corresponding locations on saidoriginal.